87 lines
2.2 KiB
Python
87 lines
2.2 KiB
Python
# Copyright 2020 The Pigweed Authors
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#
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# Licensed under the Apache License, Version 2.0 (the "License"); you may not
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# use this file except in compliance with the License. You may obtain a copy of
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# the License at
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#
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# https://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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# License for the specific language governing permissions and limitations under
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# the License.
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"""Module for low-level HDLC protocol features."""
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from typing import Tuple
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import zlib
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# Special flag character for delimiting HDLC frames.
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FLAG = 0x7E
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# Special character for escaping other special characters in a frame.
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ESCAPE = 0x7D
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# Characters allowed after a 0x7d escape character.
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VALID_ESCAPED_BYTES = 0x5D, 0x5E
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# Maximum allowed HDLC address (uint64_t in C++).
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MAX_ADDRESS = 2**64 - 1
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def escape(byte: int) -> int:
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"""Escapes or unescapes a byte, which should have been preceeded by 0x7d."""
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return byte ^ 0x20
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def frame_check_sequence(data: bytes) -> bytes:
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return zlib.crc32(data).to_bytes(4, 'little')
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def encode_address(address: int) -> bytes:
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"""Encodes an HDLC address as a one-terminated LSB varint."""
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result = bytearray()
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while True:
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result += bytes([(address & 0x7f) << 1])
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address >>= 7
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if address == 0:
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break
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result[-1] |= 0x1
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return result
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def decode_address(frame: bytes) -> Tuple[int, int]:
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"""Decodes an HDLC address from a frame, returning it and its size."""
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result = 0
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length = 0
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while length < len(frame):
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byte = frame[length]
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result |= (byte >> 1) << (length * 7)
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length += 1
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if byte & 0x1 == 0x1:
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break
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if result > MAX_ADDRESS:
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return -1, 0
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return result, length
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class UFrameControl:
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def __init__(self, frame_type: int):
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self._data: bytes = bytes([0x03 | frame_type])
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@property
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def data(self):
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return self._data
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@classmethod
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def unnumbered_information(cls):
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return UFrameControl(0x00)
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